Cocycloaddition

Using the diazadiene Fe complex 31, derived from optically active menthol, as a catalyst, enantioselective [4+4] cocycloaddition of isoprene and piperylene (30) occurred to give 3,5-dimethyl-l,5-cyclooctadine (32) with 61% ee in 89% yield without forming homodimers [10]. 

The cyclotrimerization of alkynes to give benzene derivatives is perhaps the most general reaction of these compounds in the presence of transition metal complexes. Practically any mono- or di-substituted alkyne, in addition to acetylene itself, may be cyclotrimerized. In addition, cocycloadditions involving more than one different alkyne are possible with some degree of selectivity, and intramolecular versions of the reaction have seen sophisticated development. 

Chemoselective cocycloaddition of two molecules of a terminal alkyne together with one molecule of an internal alkyne into a benzene product is possible due to the relative unreactivity of phosphine nickel carbonyls towards simple trimerization of the internal alkyne itself (equation 28). ° " Careful control of reaction conditions has also permitted selective intermolecular cocycloaddition in the presence of cobalt catalysts as well for example, diphenylacetylene and 3-hexyne give rise to a 57% yield of 1,2,3,4-tetra-pheny 1-5,6-diethylbenzene. ... 

The feasibility of carrying out chemoselective cocycloadditions by reacting a preformed metalla-cyclopentadiene and an alkyne has been explored. Despite some success in the area (e.g. equation 29) 53.74 chemoselectivity is generally compromized by the fact that the metal fragments liberated after one cocycloaddition cycle are catalytically active in promoting simple cyclotrimerization of the added alkyne. ... 

Without question, the metal-promoted cycloaddition of three alkynes to produce benzenes is the most extensively studied organometallic cycloaddition in intramolecular versions. Early work indicated the utility of Ni° systems e.g. Ni(CO)2(PPh3)2), Ziegler catalysts and rhodacyclopentadienes in the partially intramolecular cocycloaddition of a,b>-diynes with additional alkynes. Ziegler catalysts were noteworthy in giving rise to products containing the benzocyclobutene moiety from reactions of 1,5-hex-adiyne, while the Rh systems showed considerable utility in the preparation of anthraquinone derivatives from appropriate diyne precursors (Scheme 29). 

As is the case with totally intermolecular cocycloadditions, chemoselectivity is a potential problem in partially intramolecular systems where the desired process, combination of a diyne with a monoyne, may face competition from cycloadditions involving either exclusively the diyne or exclusively the monoyne. In practice chemoselectivity is readily achieved. Phosphite complexes of Ni° chemoselectively catalyze rapid and efficient cycloaddition of alkynes to heteroatom-containing diynes in good yields (equation... 

Chemoselectivity in cocycloadditions is typically poor unless the additional alkyne is resistant to simple cyclotrimerization. Examples of alkynes which satisfactorily fulfill this requirement include MesSiCBCSiMes, Me3SiC COMe and Me3SnC CSnMe3. In the case of the unsymmetrical alkyne. 

The availability of benzocyclobutenes from cocycloadditions of 1,5-hexadiynes and hindered alkynes has been elaborated into an iterative synthesis of a variety of novel polycyclic biphenylene derivatives. 

The cocycloaddition of a,d)-cyanoalkynes with alkynes is another efficient entry to bicyclic pyridines. In this process an intermediate metallacyclopentadiene is formed intermolecularly from the cyanoalkyne and the alkyne. The larger alkyne substitutent ends up nearer the metal with very high regioselectivity except in the case of 2-hexyne, where selectivity is reduced to about 3 2 (equation 43). ... 

Intramolecular cycloaddition does not occur with a,(o-dinitriles each cyano group instead undergoes independent cocycloaddition with two molecules of alkyne to afford high yields of di(2-pyridyl)alkanes. The reaction may also be controlled to give high yields of 2-((o-cyanoalkyl)pyridines. ... 

Among partially intramolecular versions of this cocycloaddition, a,o>-diynes have been found to cycloadd only with low efficiency to isocyanates in the presence of CpCo precursors. In contrast, o>-iso-cyanatoalkynes react smoothly with silylated alkynes in the presence of CpCo(CO)2, giving high yields of products formed with excellent chemoselectivity and modest to excellent regioselectivity (equation 47). The regioselectivity is most likely controlled in a manner similar to that in equation (43). ... 

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